Known to the art are internal-combustion engines for vehicles comprising a head bearing one or more cylinders, inside which the engine cycle takes place and which are set in communication with respective combustion chambers of the engine itself. Moreover provided on the aforesaid head are appropriate seats designed to enable communication of the combustion chamber with ducts designed to send into said chamber a mixture of unburnt fuel and air (intake ducts) and to remove the burnt gases from said combustion chamber (exhaust ducts).
The flows from and to each combustion chamber are controlled by appropriate valves that act on the aforesaid seats. In particular, each valve basically comprises a guide element fixed within a cavity of the engine head and a stem, which is slidably mobile in opposite directions within a through seat defined by the guide element and carries at one end an open/close portion for closing the connection between the corresponding intake or exhaust duct and the corresponding combustion chamber.
The opposite end of the stem of the valve projects axially from the corresponding guide element and is designed to receive actuation forces from a corresponding, control device, for example a camshaft.
The stem of the valve is axially loaded by a cylindrical helical spring in the direction of closing of the connection between the corresponding intake or exhaust duct and the corresponding combustion chamber.
In particular, the spring is mounted coaxially around the valve and is axially set between a fixed surface made on the engine head and a cap fixed to the stem of the valve in the proximity or at the end of the stem itself that co-operates with the control device.
Normally mounted on the valves of the type described above are seal gaskets for the lubricating oil normally circulating in engines. Said gaskets, in one of the most widely known forms, comprise a supporting or reinforcement member, which has a substantially tubular conformation and is made of a single piece of metal material, and an annular sealing element, which is made of elastomeric material and is set between the supporting member and the valve.
In particular, the sealing element typically comprises a first portion designed to co-operate, via an inner radial surface of its own, with the outer radial surface of the portion of the guide element facing in use the aforesaid control device, and a second portion designed to co-operate directly with the stem of the valve.
Gaskets of the type described above are widely used in all internal-combustion engines for control of the amount of lubricating oil that from the distribution area flows towards the combustion chambers. An excessive flow of lubricating oil causes, in addition to an evident excessive consumption of the oil itself, a deterioration of the efficiency of the engine and a reduction of the performance of the catalytic converter of the vehicle. On the other hand, an insufficient flow brings about an increase of wear and noise of the valves accompanied by the presence of local temperature peaks. These phenomena may cause a premature damage to the valves following upon seizing of the stem of the valves themselves within the guide element.
Known gaskets, via the first portion of the sealing element acting on the guide element of the corresponding valve, provide a seal of a static type, and, via the second portion of the sealing element co-operating with the stem, provide a seal of a dynamic type. In particular, the static seal must ensure a certain degree of radial compression on the guide element in order to prevent leakage of lubricating oil towards the combustion chambers and at the same time hold in position the gasket itself, whereas the dynamic seal is designed to enable the minimum flow of oil necessary for lubrication of the coupling between the stem and the guide element.
The supporting member comprises:                a substantially cylindrical main portion;        a first annular flange, which extends radially inwards from an axial end of the main portion and is in part embedded in an annular seat of the sealing element; and        a second annular flange, which extends radially outwards from an opposite axial end of the main portion and is designed to be pushed against the aforementioned fixed surface of the engine head by the spring acting on the stem of the valve.        
In practice, the second annular flange of the supporting member defines a contrast surface for an axial end of the spring and receives from the latter the normal operating loads.
The second annular flange moreover enables the gasket to be brought into action in the desired position on the valve.
There is felt in the sector the need to provide gaskets that are able to control effectively the flow of lubricating oil towards the combustion chambers and that are, at the same time, of lower cost, lower weight and greater constructional simplicity as compared to solutions of a known type, in particular in the case of use on engines of large dimensions.